In a wireless charging system, a first device may electrically charge a second device without a wired electrical connection between the two devices. For example, when the first device detects the presence of the second device, a coil in the first device may be electromagnetically energized. The electromagnetic energy from the first device may, for example, energize a coil in the second device, thus inducing an electrical current in the coil in the second device. The electromagnetic energy in the coil in the second device may then be used to charge an electric storage component, such as a battery, in the second device. Efficient transfer of energy from the first device to the second device is a goal of wireless charging systems.